The present invention relates to an autostereoscopic image-forming device comprising n viewpoints (or elementary images), said device including a lens array comprising elementary cylindrical lenses referred to as cylindrical xe2x80x9clenticlesxe2x80x9d, placed side by side parallel to a first direction.
It is known that such devices have been implemented to constitute autostereoscopic cameras, specifically the autostereoscopic picture-taking devices described in the Applicant""s following patents: U.S. Pat. No. 5,099,320, and French patents FR-2 705 007 and 2 704 951.
Those picture-taking devices implement a lens array having a large number of cylindrical lenticles placed side by side, and in practice, the video image is obtained in interleaved manner in the form of columns of pixels, the first elementary image being constituted by the pixels of the first column of pixels together with the pixels in the other columns offset therefrom modulo n, the second elementary image being made up of the pixels of the second column and of columns that are offset modulo n therefrom, and so on, each lenticle being of a width that corresponds to n columns of pixels.
The term xe2x80x9crowxe2x80x9d and xe2x80x9ccolumnxe2x80x9d respectively designate the horizontal lines and the vertical lines of pixels as seen by a standing or sitting observer, independently for example of the horizontal or vertical scanning direction of a display cathode ray tube (CRT). For example, for a CRT screen whose scan lines extend vertically, such xe2x80x9cscan linesxe2x80x9d are considered to be xe2x80x9ccolumnsxe2x80x9d in the meaning of the present text.
The quality of those picture-taking devices depends on the quality with which the lens array is made, which array has N/n lenticles, N designating the number of pixels in a video image line in the chosen standard.
As described in the above-specified French patents, an image transfer device makes it possible to use a lens array of dimensions that are about ten times greater than the dimensions of a sensor comprising a charged coupled device (CCD), thereby facilitating practical implementation.
Such a lens array thus has a large number of cylindrical lenticles, with the number of lenticles depending both on the chosen video standard (SECAM, PAL, NTSC, HDTV, etc.) and on the number of viewpoints. Unfortunately, the required accuracy of positioning is proportional to the number of lenticles.
In addition, video applications that require miniature cameras, e.g. endoscopes, require the dimensions of the lens array to be reduced very considerably, thereby making implementation much more complicated or even impossible.
An object of the present invention is thus to provide an autostereoscopic image-forming device and in particular a picture-taking device which is relatively simple to make and to adjust, and which in particular lends itself well to miniaturization, in particular for endoscopy.
The invention thus provides a device for forming an autostereoscopic image having n viewpoints (or elementary images), said device including a lens array comprising cylindrical lenticles disposed side by side and having longitudinal axes parallel to a first direction perpendicular to an optical axis of the device, the device being characterized in that it includes a cylindrical optical assembly comprising at least one cylindrical lens whose longitudinal axis is perpendicular to the first direction and to said optical axis, in that the lens array includes n cylindrical lenticles, in that the lens array and the cylindrical optical assembly share a common focusing plane corresponding to a focusing distance xcex94, and in that the absolute value of the focal length of the cylindrical optical assembly is substantially equal to n times the absolute value of the focal length of the lens array. For focusing at infinity (xcex94=∞), the common focusing plane is a focal plane common to the lens array and to the cylindrical optical assembly.
In the invention, the lens array now has only n cylindrical lenticles of pitch p, e.g. n=4, compared with the large number of lenticles implemented in the prior art picture-taking devices, i.e. N/n, e.g. 144 cylindrical lenticles for N=576 and n=4.
In addition, in the device of the invention, the stereoscopic observation baseline can be equal to twice the pitch p of the cylindrical lenticles, i.e. for n=4, to half the width L of the array. In the autostereoscopic system of the invention, e.g. having a number of viewpoints lying in the range 3 to 6, the offset between two adjacent viewpoints (or elementary stereoscopic baseline) can be equal under optimized observation conditions to half the offset E between the eyes of an observer (E=65 mm). More generally, it can be equal to half the chosen stereoscopic baseline B.
At the nominal observation distance (which is the distance at which solid color should theoretically be seen), the observer sees (for n=4) a stereoscopic pair made up in the invention by the first and third viewpoints I1 and I3, or else by the second and fourth viewpoints I2 and I4. Choosing parameters in this way so that a stereoscopic pair is made up of not of two adjacent viewpoints but of two viewpoints having an intermediate viewpoint interposed therebetween (or indeed m, where m greater than 1, intermediate viewpoints in which case the elementary stereoscopic baseline between two adjacent viewpoints is equal to the pitch p of the picture-taking lens array which is equal to B/(m+1)), thereby enabling an observer without special eyeglasses to have a viewing volume enabling the observer to move both parallel and perpendicularly to the display screen, which characterizes uniform autostereoscopy in the meaning of the present application.
As a result, and referring to the above example, an observer at the theoretical solid color distance can move towards the screen or away from it or can indeed move sideways without losing stereoscopic vision.
Once the image has been frozen on the screen, if the spectator moves towards the display screen from the nominal observation distance (or the theoretical solid color distance), the stereoscopic baseline actually seen grows while if the observer goes away from the screen, then the baseline shrinks, and the overall sensation is constant because this variation in the stereoscopic baseline compensates exactly for variation in the sensation of depth associated with modifications to the vergence force, i.e. the muscular force exerted to bring the two retinal images into correspondence so as to obtain the stereoscopic fusion that is required for relief to be perceived, which modification to vergence force necessarily accompanies displacement in the direction perpendicular to the screen.
Once the xe2x80x9csolid colorxe2x80x9d has been adjusted so that the observer sees viewpoints [(I1) and (I3)] or [(I2) and (I4)] and the observer comes close enough to the display screen, then the observer will see viewpoints (I1) and (I4) and will no longer be free to move parallel to the screen, as happens in reality when looking from close up. When the observer goes away from the screen, then the perceived viewpoints will be [(I1) and (I2)] or [(I2) and (I3)] or [(I3) and (I4)] and the observer can make large movements, thus leaving the observer free to move in a volume.
The same applies, but with improved comfort, when m is chosen to be greater than 1.
For a miniature camera or for endoscopy, the picture-taking stereoscopic baseline B is much smaller than the distance between the pupils of an observer because of the desired large magnification. For n=4, the lenticle pitch p nevertheless remains equal to half the required picture-taking stereoscopic baseline B.
Naturally this remains true for an objective lens of long focal length, for which the stereoscopic baseline B is selected to be greater than the distance E between the pupils of the observer.
In the present invention, the total stereoscopic baseline BT, i.e. the stereoscopic baseline between the viewpoints that are furthest apart is equal to (nxe2x88x921)p.
In the prior art according to the above-specified French patents, the total stereoscopic baseline is equal to the pupil diameter of the picture-taking objective lens.
In other words, other things being equal, the pitch and thus the size of the cylindrical lenticles in the present invention is N/n2 times greater than the pitch or size of the cylindrical lenticles in the prior art picture-taking devices. For n=4, they are 36 times larger than in the above-specified example (N=576 and n=4).
In a first embodiment, the cylindrical optical assembly is converging and the lens array is also converging.
In a second embodiment, the cylindrical optical assembly is converging while the lens array is diverging.
In a third embodiment, the cylindrical optical assembly is diverging while the lens array is converging.
And finally in a fourth embodiment, the cylindrical optical assembly is diverging and the lens array is also diverging.
In order to correct the anamorphosis ratio (or magnification ratio) of the system whose nominal value is equal n, but which varies slightly as a function of focusing distance, the absolute value of said ratio between the focal length of the cylindrical optical assembly and the focal length of the lenticles of the lens array is equal to n*, where n* is equal to nk, k being a magnification ratio correction coefficient such that for an object situated at a given distance, each of the viewpoints of the image has an anamorphosis ratio whose absolute value is equal to n.
In particular, and in a preferred embodiment, k varies as a function of the focusing distance and is selected so that the anamorphosis ratio is equal to n at the focusing distance.
In particular, the cylindrical optical assembly may have variable focal length. To this end it may comprise two cylindrical lenses (generally one converging lens and one diverging lens) forming a converging or diverging doublet depending on whether the cylindrical optical assembly is converging or diverging, together with means for varying the spacing of the two cylindrical lenses of the doublet so as to modify the focal length of the cylindrical optical assembly and thus modify the value of the correction coefficient k by varying the focal length of the doublet, while keeping the focusing planes of the cylindrical optical assembly and of the lens array in coincidence.
It is advantageous for the device to include a converging field lens having a focus located, for example, in the focusing plane common to the lens array and to the cylindrical optical assembly. Preferably, said field lens serves to converge the light rays that are parallel to said optical axis and passing through the centers C of the lenticles of the lens array onto a common point O advantageously situated in the entrance pupil of the transfer optical system, in other words it serves to converge the rays passing through the optical centers C of the lenticles of the lens array into the entrance pupil of the transfer optical system.
In particular, the field lens serves to avoid the phenomenon of vignetting, particularly when taking close-ups.
In a preferred embodiment, the device of the invention includes, downstream from each lenticle of said lens array, at least one plane surface that is inclined relative to a plane normal to the optical axis using angles such that light rays parallel to the optical axis and passing through the optical centers of the lenticles converge on a common given convergence point O. This serves to provide the field lens function without influencing the optical parameters of the device.
The device of the invention can constitute an element of a picture-taking device, which may be a video camera or a cinema camera, having an optical transfer system that enables said image to be centered and brought into focus.
The device of the invention, is advantageously coupled to a device for transferring the autostereoscopic image in air, including means for focusing on a video sensor such as a CCD, in particular a three-color CCD or the surface of a film.
The device of the invention can constitute a picture-taking device of a video endoscope or indeed of a videophone. With an endoscope, the image-forming device and the image-transferring video device are incorporated in the head of the endoscope. The stereoscopic baseline is generally smaller than the distance between the pupils of an observer. Video signals in xe2x80x9cn-imagexe2x80x9d mode are relayed by cable, in particular optical fiber cable, with the advantages mentioned in the above-specified French patents, so as to enable them to be viewed and/or recorded.
With a videophone, the image-forming device and the video-transfer device constitute a single picture-taking camera which relays video images in n-image mode by cable or radio to enable them to be recorded and/or displayed, e.g. on a monitor screen fitted with a display array.
It is particularly advantageous for the video image-transferring device to be a video camera such as a camcorder focused on said image. The image-forming device of the invention under such circumstances constitutes an optical accessory for the video camera to which it can be fitted by means of an adaptor ring.
In application to cinema, the device of the invention is advantageously coupled to a device for transferring said autostereoscopic image in air including means for focusing on a film. The transfer device is advantageously a cinema camera focused on said image.
The device can be characterized in that it includes a field lens, or a plane optical surface associated with each lens of the lens array, and in that said given convergence point is situated in the entrance pupil of the transfer optical system.
By applying the principle whereby light paths are reversible, the device of the invention can also be used as an element for recombining images from a projector or back-projector device whether using video techniques or cinema techniques.
The invention thus also provides a device for projecting images, either directly or by back-projection, the device including an image-forming device as defined above, said common focusing plane constituting an object focusing plane, a projector projecting an image including n flat elementary images of anamorphic format framed in the object focusing plane, and a screen fitted with at least one projection array such as a lens array or a parallax array, said projection array being situated at a distance D from the lens array of the image-forming device which is equal to the nominal viewing distance. The cylindrical lens and the lens array of the image-forming device are positioned in such a manner that the image situated in the object focusing plane is projected to the distance D by the image-forming device. The field angle of the elements of the projection array is such that each element frames the lens array of the image-forming device horizontally, and said lens array has two non-contiguous lenticles whose axes are spaced apart by the distance between the pupils of an observer.
Said screen may be a direct projection screen having a diffusing and reflecting element with the projection array placed in front thereof, which array also constitutes a viewing array.
Said screen may be a back-projection screen including a projection array and a display array, with a diffusing element being disposed between them.
Finally, the invention provides an autostereoscopic video system including a picture-taking device as defined above and a projection device as defined above.